1. Field of the Invention
The present invention relates to a spectrum spread communication synchronization establishing apparatus for a receiving section of a radio apparatus for a spectrum spread communication.
2. Description of the Related Art
A receiver of a direct spectrum spread system basically receives a spectrum spread signal by an antenna to convert to an intermediate frequency signal or baseband signal. Then, the receiver establishes synchronization with a spreading code signal used in a transmitter by the synchronization establishing circuit and sends the synchronized spectrum spread signal to a despreading demodulator. The despreading demodulator carries out inverse modulation. That is, the despreading demodulator multiplies the synchronized spectrum spread signal by the spreading code. A data demodulator demodulates the multiplication result to produce a data.
The synchronization establishing circuit of the receiver needs to carry out a searching operation of a phase coincident point with the spreading code and to restrict a timing in a predetermined range for establishing and tracking synchronization.
FIG. 1 shows a conventional spectrum spread communication synchronization establishing apparatus. Referring to FIG. 1, the conventional spectrum spread communication synchronization establishing apparatus is composed of a plurality of synchronizing circuits 400. Each of the synchronizing circuits 400 is composed of a local oscillator 1, a multiplier, a low pass filter (LPF) 2, a sampling and holding circuit (S/H) 3, a correlating unit 4, and a symbol integrating unit 8. A signal converting section is composed of the local oscillator 1, the multiplier, and the low pass filter (LPF) 2 and converts a received signal from a corresponding antennal into a baseband signal. The sampling and holding circuit (S/H) 3 samples the baseband signal to store and hold and outputs a sampling signal. The correlating unit 4 calculates correlation between the sampling signal and a spreading signal of a spreading code to produce a correlation value. The symbol integrating unit 5 multiplies the correlation value by a theoretical value of a symbol corresponding to the correlation value in case that the symbol is known or a determination value after demodulation in case that the symbol is unknown. Then, the symbol integrating unit 5 adds the multiplication results for a plurality of symbols, and calculates power of the addition result for the plurality of symbols to produce a power value.
The spectrum spread communication synchronization establishing circuit is further composed of a plurality of path search sections 10 respectively provided for the synchronizing circuits 300 and a demodulation path selecting section 12. Each of the path search sections 10 adds the power values for a plurality of slots to produce a power addition value, and selects larger ones of the power addition values in order to outputs timings corresponding to the selected power addition values. The demodulation path selecting section selects a demodulation reception timing from the timings outputted from the path search sections 10 based on the selected larger power addition values and outputs a demodulation path data.
In the spectrum spread communication synchronization establishing apparatus, a spectrum spread signal is received by the antenna, and then is converted into the baseband signal by the local oscillator 1, the multiplier and the low pass filter (LPF) 2. Subsequently, the sampling and holding circuit (S/H) 3 samples the baseband signal for every ½ chip and supplies the sampled signal to the correlating unit 4 via a memory.
The correlating unit 4 is generally composed of a matched filter. The correlating unit 4 multiplies the spreading code for one symbol of the sampled signal read out from the memory by the preset spreading code for one symbol for every chip and outputs a summation of the multiplication results as a correlation value. This operation is carried out at a plurality of sample points, i.e., a plurality of paths which are shifted by ½ chip, 1 chip, 3/2 chip, and so on. The respective correlation values are outputted. Therefore, the correlation values corresponding to the plurality of paths are outputted from the correlating unit 4.
In the symbol integrating unit 5, a symbol signal obtained by carrying out a despreading operation in the interval of ½ chip is inversely modulated. That is, the symbol signal is multiplied by a theoretical value for a symbol in case the symbol is known in a transmission sequence or a determination value determined after demodulation in case that the symbol is unknown. Then, the multiplication results are added for a plurality of symbols for every path so as to carry out symbol integration. It should be noted that the whole or a part of the symbol contained in a slot is used for the symbol integration. The power value of the integrated addition value is determined and outputted to the path search section 10.
The path search section 10 adds the power values for a plurality of slots to produce a power addition value. Subsequently, the path search section 10 selects larger ones of the power addition values in order to outputs the selected power addition values together with timing data. The demodulation path selecting section 12 selects some path timings based on the timing data and the selected larger power addition values outputted from the path search sections 10. The demodulation path selecting section 12 outputs the timing of the path and antenna data to a demodulating section which is connected with the demodulation path selecting section 12.
As methods for selecting the path in the demodulation path selecting section 12, there are a method for allocating demodulation paths in order of larger average power per one slot, a method for weighting the power addition value for each path in accordance with the signal to interference ratio (SIR) of the path and allocating the demodulation paths in order of larger weighted power addition values, and a method for selecting the path in consideration of past changes of the power addition values of each path. In either of the methods, any one of the paths is selected when the path timings are close to each other in case of paths obtained from the same antenna.
According to the above-mentioned conventional spectrum spread communication synchronization establishing apparatus, an appropriate synchronization timing can be established with less erroneous determination of a synchronization position. Also, even when the synchronization timing is changed, the change is able to be tracked. Thus, the receiving quality can be improved and it is not necessary to use wasteful strong transmission output on the transmission side in order to attain a predetermined receiving quality, resulting in suppression of interference.
On the other hand, in this kind of receiver, there is a frequency offset generated in the transmission path due to the Doppler effect or a frequency offset due to a frequency error between a receiver oscillator and the transmitter oscillator. If the frequency offset exists, a gain cannot be sufficiently obtained due to phase rotation caused by the frequency offset in in-phase addition of outputs from the correlating units 4 for several symbols.
In addition, when the frequency offset increases, the sufficient gain cannot be obtained, and conversely, the addition may exert detrimental effects. Consequently, the number of symbols for the in-phase addition must be decreased not to receive influence of the frequency offset.
In conjunction with the above description, a search and establishing method of a code division multiple access signal is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-313382). In this reference, a code division multiple access signal is received, and a spreading code signal is extracted from the received code division multiple access signal. A local spreading code signal is generated and a frequency offset is detected between the extracted spreading code signal and the local spreading code signal. Whether the received code division multiple access signal is appropriate is determined based on the frequency offset. The received code division multiple access signal is decoded when it is appropriate.
Also, a spectrum spread type receiver is disclosed in Japanese Patent No. 2672769. In this reference, a quasi-synchronization detecting circuit mixes a received spectrum spread (SS) signal which has been spectrum spread by use of a pseudo-noise (PN) signal to a local carrier orthogonal to the SS signal and gets a complex baseband signal. A partial correlation calculation section divides the complex baseband signal into a plurality of partial data and calculates correlations between these partial data and corresponding partial PN signals to produce partial correlation signals. An absolute value square summation calculating section calculates a summation of squares of absolute values of the partial correlation signals and outputs a correlation absolute value square summation signal. An initial synchronization establishing and tracking section detects a repetition period of the PN signal contained in the received SS signal based on the correlation absolute value square summation signal and output a timing signal synchronized with the repeating period.
Also, a spectrum spread type receiver is disclosed in Japanese Patent No. 2698506. In this reference, a spectrum spread type receiver receives a direct spectrum spread (SS) signal spectrum spread by a pseudo-noise (PN) signal. A quasi-synchronization detection circuit mixes a reception SS signal and a local carrier to produce a complex baseband signal. A correlating unit carries out an operation of the correlation between the complex baseband signal and the PN signal. An AFC circuit generates an error signal in accordance with a frequency offset of the local carrier to the reception SS signal and corrects the influence of the frequency offset of the local carrier in accordance with this error signal. An initial synchronization establishing and tracking circuit carries out initial synchronization establishment and tracking of the PN signal based on a result obtained by adding a signal of the magnitude of the correlation signal and a signal of the magnitude of the error signal.
Also, a spectrum spread communication synchronization establishment demodulator is disclosed in Japanese Patent No. 2850959. In this reference, a spectrum spread code synchronous circuit carries out demodulation after despreading with a spreading code, when a spectrum spread signal obtained by spectrum spread using the spreading code after modulating a transmission data and then transmitted is received as a reception signal, and demodulated so as to reproduce a reception data. A signal converting section converts the reception signal into a baseband signal. A sampling and holding circuit samples, stores and holds the baseband signal and outputs a sampling signal. A first correlating unit calculates correlation between the sampling signal and a spreading signal of the spreading code to produce a first correlation value. A symbol integrating unit inversely modulates the first correlation value based on either of a theoretical value of a symbol corresponding to the first correlation value in case of the known symbol or a determination value after demodulation in case of unknown symbol, and adds the inverse modulation result for a plurality of symbols, and calculates power of the addition result to produce a power value. A short-time integration path search section adds the power values for a plurality of slots and selects ones of the addition results in order of larger power value per a slot for the number of first correlating units. A long time integration path search section adds the power value for a plurality of slots longer in time than that of the short time integration path search section and selects ones of the addition results in order of larger power value per a slot for the number of first correlating units. A demodulation path selecting section selects demodulation reception timings from the timings from the short time integration path search section and the long time integration path search section in order of larger power value per a slot. A second correlating unit calculates correlation between the reception signal and the spreading signal at the demodulation reception timings to produce a second correlation value. A detector detects the second correlation value and outputs a detection signal. A signal synthesizing section outputs the determination value based on a synthesis signal obtained by carrying out RAKE synthesis and space diversity synthesis of the detection signal for each path.